This invention relates to a safety radiator cap, in particular, for use in automobiles. The safety radiator cap provides a safety plate or wire made of a shape memory alloy. The shape memory effect of the shape memory alloy was known about 1950. It was found that the reversibility of the martensite transformation is the principal reason for the characteristic of shape recovery. The shape memory alloy appears as full martensite phase at temperatures below a critical temperature called martensite finishing temperature (M.sub.f in abbreviation) and becomes as full austenite phase after being heated above another critical temperature called austenite finishing temperature (A.sub.f in abbreviation). The stiffness of a shape memory alloy at austenite phase is much stronger than that at martensite phase. For example, the yield strength of a Ni-Ti shape memory alloy at austenite phase in general is four times of that at martensite phase. When, after running the engine, the water in the radiator becomes hot, the safety plate extends out due to its shape memory characteristic as soon as the temperature beyond A.sub.f. When someone wants to open the radiator cap to add water into a hot radiator, the safety plate will press against the neck of the filling hole and quickly exhaust the steam. This thus prevents scalding with hot steam which might otherwise spray out from the filling hole of the radiator.
Although automobiles are the common vehicles for road transportation, we still often hear about drivers being scalded with hot steam due to their carelessness. We know that radiator water gradually dissipates while driving, and it is periodically necessary to add water. Although some radiators provide an overflow pipe for hot steam to escape, it cannot exhaust the hot steam completely in a short time and the steam left in the radiator is still very hot. When someone hurriedly opens the radiator cap, the steam sprays out and scalds him or her. The present invention overcomes this disadvantage of conventional radiator caps.